regulator: Improve WM831x DVS VSEL selection algorithm

Rather than using the maximum voltage we get passed to select the DVS
voltage to use remember the highest voltage we've ever seen. This improves
how the driver works when the consumer permits higher voltages than it
will ever selects in order to support the widest possible voltage range.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>